Moon powered monsters

TLDR: imagining spirit-monsters that are bound by circadian rhythms.

The Bakemono Zukushi Scroll
Check out this Edo-period Japanese Monster Manual ... er, I mean painted scroll featuring shape-changing Bakemono. The artist and date is unknown, though it is thought to be from the 18th or 19th century.

A portion of the Bakemono Zukushi Scroll

One of the great things that comes out in the discussion of this scroll is the way whole classes of monsters are distinguished by the time of day in which they are active.

The founding father of minzokugaku (Japanese folklore studies), Yanagita Kuno (1875–1962), drew a distinction between yurei (ghosts) and bakemono: the former haunt people and are associated with the depth of night, whereas the latter haunt places and are seen by the dim light of dusk or dawn.

It reminds me of the three spirits that visited Ebenezer Scrooge in Dickens' Christmas Carol, each one appearing only when the other had disappeared. Of course they represented a sort of chronology (past, present, future) and they could only appear on Christmas night. But the association for me is that gateways to the spirit world may operate a bit like a time-lock vault that is regulated by the calendar, the moon, or the day/night cycle. Certain monsters can only appear when the stars are right. Weather, season, light level, etc. can all be predictors of monsters appearing. They might even have names like mist-walkers, snow walkers, or moon beasts.

Circadian rhythms would be a really interesting twist to put on creatures in your RPG adventures. The characters might be in an otherwise safe place, and then twilight comes, or the witching hour, and things get dicey. This idea also works as a pacing mechanic. In the "off-hours" characters could run around in relative safety, trying to find some formula or weapon to use against the creature(s), but the clock is ticking and they must assemble the right things and perhaps even be at the right location to drive off or destroy the monster(s).

Imagine a villain who is invincible except at sunrise and sunset. Or malformed spirit creatures that can only break through to the material world when someone harbors violent thoughts at midnight.

Another parallel, in my mind, is the play of light and dark in The Lord of the Rings. Orcs went all weak-in-the-knees in sunlight, so there was a measure of safety while during the day – or at least while abroad under a sunny sky. At night you wanted to be behind fortified walls if you could. And, I believe it is Gandalf who says "look for my coming at first light" and Aragorn who calls down to the Uruk Hai "None knows what the new day shall bring him ... Get you gone, ere it turn to your evil." Of course that sense of safety was soon to be eroded by the blanket of dark clouds Sauron sent forth to shield his troops. Edit – given that spells are often not given a termination clause in older D&D, I have toyed with the idea that all spells break (or at least afford a saving throw to those affected) at dawn.

There's a lesson to be had there. Once the characters figure out that the evil sorcerer comes at twilight, because that is when his powers are strongest, how can you surprise them? How could you artificially induce twilight? An eclipse, perhaps?

There's a lot of meat on this bone. And a lot of cool ideas for creatures embedded in that scroll ... not to mention some ready-made, copyright free illustrations!